1. Field of Invention
The present invention relates to a method and apparatus for controlling an assembly of an electrical motor and a rotation-translation converter connected thereto.
2. Description of Related Art
U.S. Pat. No. 5,678,671 discloses a motor vehicle clutch assembly having an actuator device, the actuator device having a clutch motion blocking system. The clutch motion blocking system may be a current supply system, supplying a blocking current to the actuator device to block the linear movement of the actuator in a preferred position. The blocking current is specified as being less than 10 Amperes, and preferably less than 6 Amperes.
German patent DE-C-196 01 983 discloses a brake system for vehicles with an integrated electrical parking brake. The electrical parking brake is actuated with a current supply when driving the vehicle such that the electrical parking brake exerts no force. By cutting the current to the electrical parking brake, the friction of the brake system is enhanced resulting in a self-arresting brake action of the brake system.
U.S. Pat. No. 4,033,435 discloses a retainer mechanism for vehicle brake actuators. An electrical motor is operable when energised to hold a resilient brake applying assembly in a retracted energy-storing position and to retain the assembly in the retracted position. In this system, the current to the electrical motor has to be increased to decrease the brake action.
Conventionally, in controlling assemblies of an electrical motor and a rotation-translation converter, as known by the person skilled in the art, the electrical motor is controlled for providing a torque, such that the rotation-translation converter exerts a predetermined force on an object attached to the rotation-translation converter, which cannot move further in the direction of the exerted force.
An Electro Mechanical Brake Actuator (EMBA) is described in Dutch patent application 1009626, in the name of the applicant, which is hereby incorporated by reference. In these brake actuators, a predetermined force has to be exerted on a brake piston during a certain time period (on the order of 1 sec . . . 20 sec). This is achieved by controlling the current through the electrical motor of the brake actuator. The electrical motor provides a torque, which in first instance is converted by a rotation-translation converter into a movement of the brake piston attached to the output side of the rotation-translation converter until it touches the brake lining or brake disc. As the brake piston is unable to move further, the torque of the electrical motor is converted into a force on the brake piston, resulting in a brake action. Preferably, this control is utilised for static forces which need to be maintained during a certain time period (1 sec . . . 20 sec) and is only applied after, e.g., 0.5 sec.
However, the rotation-translation converter has an internal friction which needs to be overcome by the torque provided by the electrical motor. This requires an initial current. Only when, after this, the current through the electrical motor is further increased, a translational movement at the output side of the rotation-translation converter will result, or, when the brake piston touches the brake lining or brake disc and can not move further, a force will be exerted on the brake piston. It is also possible that the moveable object is a spring (in case of the EMBA a brake calliper) with a certain rigidity, whereby the force on the spring will be accompanied by a predetermined displacement. The brake force of the EMBA will now increase approximately linearly with the current fed to the electrical motor.
When it is required to decrease the predetermined force (ending or reducing the brake action), the current first has to decrease to a predetermined current value, whereby the friction of the rotation-translation converter has to be overcome in the opposite direction. During this reduction, the force exerted on the brake piston remains substantially the same. When the friction force of the rotation-translation converter has been overcome in the opposite direction, the force on the brake piston at the output side of the rotation-translation converter decreases with the decreasing current through the electrical motor. When no resulting force is exerted on the brake piston, it will move away from the brake lining or brake disc, ending the brake action.
The internal friction of the rotation-translation converter therefore results in a hysteresis in the relationship of the current through the electrical motor and the force exerted on the brake piston by the rotation translation converter.
When controlling the EMBA, it happens that the brake force has to be maintained during a predetermined time period (on the order of 1 sec . . . 20 sec). A disadvantage of the known method for the operation of e.g. the EMBA, is that actually too much current is used in maintaining a predetermined brake force. This leads to firstly, a larger use of energy and secondly, development of extra heat in the EMBA.
In one aspect, the present invention relates to a method for controlling an assembly of an electrical motor and a rotation-translation converter which uses (electrical) energy more efficiently than the known systems. The method has the advantage that during the maintaining of the predetermined force at the output side of the rotation-translation converter, less current is required to flow through the electrical motor, resulting in less energy consumption of the assembly. This saving of energy is accompanied by a reduced heat development. These mentioned advantages will also make the operation of the assembly more reliable and the lifetime of the assembly will increase.
From the specifications of the rotation-translation converter, e.g., the pitch and diameter of a threaded converter, the manner in which the conversion of the torque of the electrical motor in a force on the object takes place, can be deduced quantitatively. As the current-torque characteristics of the electrical motor are also known, it is simple to determine the relationship of the current through the electrical motor and the force on the object in the ideal case (i.e., without friction). When it is required to maintain a predetermined force on the object, it is therefore also known what current should flow through the electrical motor. The current can be decreased until that current value, without reduction of the force on the object.
When measurement means are provided for detecting at which current the internal friction of the rotation-translation converter is overcome, the value of the first current can be determined as the difference of the current at which the predetermined force is reached and the current at which the internal friction is overcome.
Because the rotation-translation converter has an internal friction, the current can actually be reduced even further as in the embodiment described above. The torque of the electrical motor will have to be reduced until the converted torque plus the friction force of the rotation-translation converter is equal to the force exerted on the object. Only a further reduction of the torque of the electrical motor will result in a decrease of the force on the object.
In a preferred embodiment of the present invention, in the step of reducing said current, said current is further reduced to a transition current value at which the torque delivered by the electrical motor and converted by the rotation-translation converter into a force plus the internal friction of the rotation-translation converter is equal to the predetermined force on the object.
As the force-current relationship is symmetric, the transition current value can also be determined when, as described above, the current is known in the ideal case without internal friction of the rotation-translation converter. The transition current value is then equal to twice the current in the ideal case minus the current at which the predetermined force is reached.
In order to optimally use the energy to energise the electrical motor, attempts will have to be made to attain as close as possible the current value at which the torque delivered by the electrical motor and converted into a force by the rotation-translation converter plus the internal friction of the rotation-translation converter is equal to the predetermined force on the object. This current value constitutes the transition point, at which a further reduction of the current through the electrical motor will result in a reduction of the force on the object.
In a further embodiment of the method according to the present invention, the method, therefore, comprises the further step of determining a minimum current value by decreasing the current through the electrical motor stepwise until a decrease of the force on the object or a backward movement of the rotation-translation converter is detected.
The minimum current value can be detected by observing the force on the object, or by observing a backward motion (at the rotation or translation side) of the rotation-translation converter.
A further embodiment of the method according to the present invention comprises the further step of again increasing the current through the electrical motor.
By adjusting the current to a value slightly higher than the transition current value, further reduction of the force by external influences is prevented. When the friction force of the rotation-translation converter decreases or the force on the object increases as a result of e.g. oscillations of the brake disc or variation in thickness, the transition point will not be passed when the current is kept equal.
A still further embodiment of the present method comprises the further step of again increasing the current through the electrical motor until the predetermined force has been reached again and successively decreasing the current to a level which is at least one current step higher than the previous determined minimum current value.
In this embodiment, therefore, the method returns to a level of the force on the object and the current just before the reversal point, making the method even less sensitive to external influences.
In another aspect, the present invention relates to an apparatus comprising an assembly of an electrical motor and a rotation-translation converter connected to said electrical motor, an object moveable by said rotation-translation converter, control means for driving said electrical motor with a current (I) and first detection means connected to said control means for detection of a force (F) exerted by said rotatio-translation converter on said object, said control means (6) being equipped to control said current (I) through said electrical motor by energising said electrical motor with a current (I) having a first value (I1) for obtaining a torque overcoming the internal friction of said rotation-translation converter, increasing said current (I) until said electrical motor provides a total torque, which is converted by said rotation-translation converter into a predetermined force (F1) on an object moveable by said rotation-translation converter, reducing said current (I) through said electrical motor after reaching said predetermined force (F1), during which said predetermined force (F1) on said object is maintained substantially equal.
The first detection means, e.g., measure the force which the rotation-translation converter exerts on the object, by means of which the control means adjust the current through the electrical motor. However, the first detection means can also detect the force on different locations and in different ways.
In a further embodiment, the apparatus further comprises second detection means connected to said control means for detection of rotation of said rotation-translation converter. This embodiment has specific advantages. When an equally maintained force is exerted on the object by the rotation-translation converter, the rotation-translation converter will of course not rotate. The second detection means can detect whether the torque, converted into a force by the rotation-translation converter, plus the friction force of the rotation-translation converter becomes smaller than the predetermined force, as in that case the rotation-translation converter will rotate in the opposite direction.